DECARBOXYLATION RATE OF URONIC GROUPS CONTAINED IN SOIL ORGANIC MATTER, PLANT GUMS OF KNOWN CONSTITUTION, PLANT MATERIALS, AND MICROBIAL PRODUCTS

Abstract

The rate of evolution of CO2 from soil organic matter and various uronic-containing substances, when boiled with 12% HC1, was followed in order to obtain evidence as to the nature and source of the CO2-yielding constituents of soil organic matter. The CO2 evolution from both surface soil and subsoil was greatest during the first 15 min., then decreased abruptly and continued at a slowly decreasing rate during the 7-hr. digestion period. 9%-23% of the organic C in the surface soils appeared to be uronic; the corresponding value for the subsoil was 19 to 30%. The rate of CO2 evolution from lemon pectin and from mesquite gum was a little less than from soil organic matter but otherwise similar. The uronic units of both natural and pectin-extracted plant materials were decarboxylated at about the same rate as those of plant gums. A much smaller percentage of the total C of plant materials was present as uronic C than in the case of soil organic matter. Biological decomposition of plant materials resulted in a rapid loss of uronic C during the first 5 days but after 6 months many plant residues contained more uronic units than after 10-30 days, even though the total wt. of the sample was much lower. Uronic C was undoubtedly synthesized by microorganisms. Decarboxylation rate studies with the gums produced by 2 typical soil organisms, Rhizobium trifolium and Sporocytophaga myxococcoides, gave rate curves similar to those for soil organic matter.